have one's hand in the till

Francis Urquhart: You've been a bit of a bad lad, haven't you, Roger?

Roger O'Neill: What?

FU: You've had your hand in the till.

RO: Francis, you're joking!

FU: Your expenses are paid by Central Office, aren't they? But you've been claiming quite large sums from the advertising agency as well. It's a bit naughty, Roger.